High speed chain or belt printers for computer print-out and the like often employ type slugs attached to an elastomeric timing belt wherein the teeth of the timing belt provide accurate locating means for the type slugs. A high speed printer in accordance with this concept is shown and described in U.S. Pat. No. 3,621,778. In this patent an elastomeric timing belt having accurately spaced teeth along its inner surface carries a plurality of type slugs clipped over the upper and lower edges of the belt and secured and accurately located each by two pins top and bottom each two pins gripping a tooth of the belt. The belt passes over two pulleys one of which is a drive pulley and the other an idler pulley for maintaining the belt under longitudinal tension. The pulleys carry peripheral slots shaped and spaced to accurately mate with the ribs of the belt. The area between the slots which could be considered to be the pulley teeth ride against the belt between the teeth.
Now the pins which secure the slugs on the belt overlap the inside of the belt top and bottom and provision must be made so that these pins do not ride on or come in contact with the surface of the pulleys. In the past this has been accomplished by limiting the width of the pulley slots and ribs to the inside distance between the slug holding pins as can be seen in FIG. 1 of U.S. Pat. No. 3,621,778 referenced above. This construction, however, leaves the edges of the belt for a distance equal to the slug mounting pin overlap unsupported as the belt goes around the pulleys. This lack of support along the edges of the belt and in the area of the slug mounting pins has been found to cause distortion of the edges of the belt and displacement of the slugs mounted along these edges. It will be appreciated that dynamically the belt is moved very fast along the print line and the slugs are subjected to rapid impact by the printing hammers. This high speed impact causes tension waves along the edges of the belt and with insufficient support in the area of the pulleys permits lateral motion and misalignment of the characters printed from the slugs.
In accordance with the present invention an additional set of ribs is provided at the top and bottom of the pulleys designed and positioned to provide support to the belt along the previously unsupported top and bottom edges. The ribs on these auxiliary pulleys have radii equal to the radii of the teeth of the main pulley, are equal in number to the ribs of the main pulleys; and are positioned to fall between the pins of adjacent slugs. The slots between the ribs of these auxiliary pulleys are wide enough to clear the two mounting pins of each slug and including the teeth of the belt embraced by each set of two pins.